Conventional motors include delta wound three-phase permanent magnet brushless motors. Conventional motor controllers for delta wound three-phase permanent magnet brushless motors monitor current for the motor based at least on a measured phase current, wherein the measured phase current is derived from using a single current sense resistor. See U.S. Pat. No. 6,262,544. The motor controller uses the measured phase current to control the amount of current supplied to the motor. However, after each motor commutation, the current measurement derived from a single current sense resistor provides an inaccurately low measured phase current due to recirculating currents in the motor windings. The winding topology of a delta wound three-phase permanent magnet brushless motor allows the current in at least one of the motor phases to improperly increase during the period of inaccurately low measured phase current. Such motor control has a tendency to create a field in a phase winding strong enough to demagnetize the permanent magnets of the rotor of the brushless motor and strong enough to cause excessive thermal stress in the motor and the motor controller when the motor operates in the second and/or fourth quadrants of a motor speed versus motor torque curve of the motor.
What is needed is an improved method for operating a delta wound three-phase permanent magnet brushless motor using a single current sense resistor.